1. Field of the Invention
The present invention generally concerns expanding gate valves and more particularly concerns expanding gate valves having gate and gate expansion components that can be more easily opened and closed as compared with more conventional expanding gate valve mechanisms. More particularly, the present invention concerns a valve mechanism having a pair of gate members and a valve stem arrangement that permits sequential initial opening movement of one of the gate members prior to initial opening movement of the other gate member to ensure opening of the expanding gate assembly by application of minimal force. This invention also concerns an expanding gate valve mechanism having upstream and downstream gate members and a gate expansion wedge for gate expansion and having a gate actuator mechanism ensuring initial opening movement of the upstream gate member prior to initiation of opening movement of the downstream gate member. The present invention concerns an expanding gate valve mechanism that effectively prevents the occurrence of the well-known problem of back-wedging or premature expansion of an expanding gate assembly during linear closing movement thereof.
2. Description of the Prior Art
One popular type of expanding gate valves were initially developed by Milton P. Laurent in the mid-1930's, with U.S. Pat. No. 2,148,628 being issued to Laurent in early 1939. This particular type of expanding gate valve has been quite popular in the petroleum and other similar industries where efficient and effective control of high pressure fluids is required. The expanding gate assemblies of these types of expanding gate valves generally comprise a pair of gate members each having flat sealing surfaces for sealing with spaced seat surfaces within the valve and each having an angular gate expansion face. Typically, only one of the gate members is directly driven be a valve stem, while the other gate member is somewhat free floating and is moved along with the driven gate member. When the linear travel of the floating gate member is stopped by an internal stop surface or stop member within the valve body the stem driven gate member is further linearly moved by a gate actuator mechanism. This relative linear movement of the gate members causes camming interaction of the angular faces of the gate members, thus causing lateral expansion movement of the gate members, resulting in forcible movement of the gate members into sealing engagement with internal seat surfaces within the valve body. Increase of the closing force of the valve stem achieves corresponding increase of the sealing force of the gate members with the internal seat surfaces within the valve body.
A principal disadvantage of conventional expanding gate valves is that the gate and gate expansion geometry often fail to permit significant collapse of the gate mechanism during gate unsealing and opening movement. This condition causes the gate members to slide along the seat surfaces within the valve during valve opening movement, causing significant wear and erosion of the seat surfaces or seat assemblies as well as wear and erosion of the sealing surfaces of the gate members. It is desirable therefore, to provide an expanding gate valve mechanism that will readily become collapsed upon initial opening movement of the gate mechanism, thus minimizing the seat and sealing surface wear and erosion that might otherwise occur.
A problem that exists with most conventional expanding gate valves is the potential for back-wedging or premature wedging. This is the occurrence of mechanical wedging and gate expansion before the gate members have reached their completely closed positions. As the gate assembly of an expanding gate valve is moving toward its closed position, the force of fluid pressure acting on the gate mechanism can develop sufficient drag or resistance on the gate members that cause relative gate movement, resulting in mechanical gate expansion before the gate mechanism has reached its closed position between the valve seats. When premature wedging of the gate and gate expansion mechanism occurs the gate mechanism will experience significant drag, resulting in resistance of the gate closing and sealing activity. This premature gate expansion develops enhanced drag or resistance to gate movement, which increases the gate expansion force, and oftentimes causes the gate mechanism to become stalled or locked before reaching its closed position so that it cannot be completely closed. Obviously, the frictional resistance of premature expansion of an expanding gate mechanism causes significant wear of the gate and seat surfaces of the valve, adding significantly to the maintenance costs of the valves. Premature wedging can occur when flow is in the non-preferred direction which is possible in many valve applications. Back-wedging of expanding gate valves is conventionally prevented by lever mechanisms as shown in U.S. Pat. No. 5,743,288 of Mosman et al, cams as shown in U.S. Pat. No. 6,164,622 of Partridge, springs, as shown in U.S. Pat. No. 2,148,628 of Laurent or combinations of these features.
Undesirable back-wedging or premature wedging activity during valve closing is prevented by the present invention because both gate members are supported by the valve stem during both opening and closing movement of the expanding gate mechanism. The wedge member has no external forces acting on it due to pressure or operation. Force is only applied to the wedge member when the wedge is moved into contact with the stop and the gate members are moved relative to the wedge member by the valve stem. The expanding gate mechanism is virtually at its closed position when the wedge member comes into contact with the gate stop. And when the wedge member of the preferred embodiment has become stopped the gate forces act in substantially equally balanced manner on the tapered surfaces of the wedge member.
An advantage of moving gate members laterally into sealing engagement with seat surfaces is that the gate members do not tend to drag or slide along the seat surfaces with high friction engagement and thus the gate and seat surfaces within the valve are not subjected to excessive wear during opening and closing movement of the expanding gate mechanism. Consequently the sealing surfaces of the valve seats and gate members do not tend to become rapidly worn or eroded during frequent valve operation under high pressure conditions.